The present invention relates to a misfire detecting device for an internal combustion engine, and more particularly to a misfire detecting device for an internal combustion engine having enhanced reliability in the entire operating area by detecting a misfire on the basis of the rotation information or an ion current in an operating area.
Conventionally, the misfire detecting device for the internal combustion engine employing a variation in pulse period ratio as the rotation information was well known. Further, the misfire detecting device for the internal combustion engine employing a detection level of the ion current was also well known.
Generally, it is well known that the reliability (S/N ratio) of misfire detection based on the rotation information and the ion current changes in accordance with the operating area (rotation number) of the internal combustion engine, and the reliability of misfire determination based on the period ratio (rotation information) is degraded with an increased rotation number of the engine. Also, the reliability of misfire determination based on the ion current is degraded with a decreased rotation number of the engine.
Thus, in order to solve the above-mentioned problems, a technique has been proposed in which the reliability of misfire determination in the entire operating area is improved by detecting a misfire on the basis of the rotation information in a low rotation area and detecting the misfire on the basis of the ion current in a high rotation area, as disclosed in the Unexamined Japanese Patent Application Publication No. 2000-240550.
However, in a multiple cylinder engine (e.g., 8 or greater cylinder engine), the reliability of misfire determination in the low rotation area may be degraded even on the basis of the rotation information. In the conventional misfire detecting device for the internal combustion engine as above mentioned, the misfire detection is only based on the rotation information in the low rotation area, resulting in a problem that the reliability of misfire determination is degraded to lead to a false determination of misfire during the normal combustion.
This invention is achieved to solve the above-mentioned problem, and it is an object of the invention to provide a misfire detecting device for the internal combustion engine in which the reliability of misfire determination in the low rotation area is improved.
According to a first aspect of the present invention, there is provided a misfire detecting device for an internal combustion engine comprising various kinds of sensors for sensing an operating condition of the internal combustion engine, operating area detecting means for detecting the operating area of the internal combustion engine from the operating condition, control parameter calculating means for calculating a control parameter of the internal combustion engine, on the basis of the operating condition, an igniter for igniting the internal combustion engine in accordance with the control parameter, an ion current detector for detecting an ion current produced by ignition of the internal combustion engine, first misfire determination means for determining a misfire of the internal combustion engine on the basis of a temporal change of the rotation information of the internal combustion engine contained in the operating condition, second misfire determination means for determining a misfire of the internal combustion engine on the basis of the ion current, and misfire determination selecting means for selectively employing the first or second misfire determination means in accordance with the operating area, characterized in that when the operating area detecting means detects the operating area that is less than or equal to a predetermined rotation number, the misfire determination selecting means determines the misfire in accordance with the logical product between a detection result of the first misfire detecting means and a detection result of the second misfire detecting means.
Also, according to a second aspect of the invention, there is provided the misfire detecting device for the internal combustion engine, characterized in that the misfire determination selecting means comprises ion current state determining means for determining an adequate state of the ion current, wherein when it is determined that the ion current is adequate, means for determining the misfire in accordance with the logical product between a detection result of the first misfire detecting means and a detection result of the second misfire detecting means is employed in an area of a predetermined rotation number or less, while when it is determined that the ion current is inadequate, the first misfire detecting means is employed in the area of the predetermined rotation number or less.